1. Field of the Invention
This invention relates to a process for preparing a block copolymer compostion, and more particularly, to a process for preparing a block copolymer composition comprising a linear triblock copolymer in the type of vinyl aromatic block-conjugated diene block-vinyl aromatic block and a diblock copolymer in the type of vinyl aromatic block-conjugated diene block, said process being characterized in that some parameters such as diblock content (weight % of diblock copolymer fraction in the total block copolymer), respective molecular weight and chemical composition of triblock and diblock may be easily controlled via appropriate control of termination steps, thus obtaining desired physical properties.
2. Background of the Invention
When a block copolymer composition comprising a triblock copolymer in the type of vinyl aromatic block-conjugated diene block-vinyl aromatic block and a diblock copolymer in the type of vinyl aromatic block-conjugated diene block is intended for an adhesive use, the diblock content needs to be variously controlled depending on its use.
In applications where said block copolymer composition is used as a base resin in adhesive tapes, diblock content of the base resin controls the cohesive strength and wettability of the tape. (Polym. Prep. 1996, 37(2),720).
Accordingly, diblock content should be adjusted in an appropriate manner depending on the adhesive properties required.
As mentioned in the following there are some conventional methods which provide control of diblock content in a block copolymer composition.
The U.S. Pat. No. 4,096,203 disclosed a method to control diblock content based on appropriate control of coupling efficiency of a living diblock copolymer in the type of vinyl aromatic block-conjugated diene block-Li.
However, such method has faced some shortcomings in that a) since it is practically impossible or very difficult to freely control the coupling efficiency over 90%, especially over 95%, diblock content can be controlled in a limited range, b) preparation of a block copolymer composition with high diblock content such as 30 to 70 wt % needs a small mole percent of a coupling agent relative to the living diblock copolymer, leaving considerable amounts of a living polymer after the coupling reaction, requiring additional termination step which necessitates additional process chemicals and facilities, c) in case that some commonly available halogenated compounds are employed as coupling agents to synthesize a linear triblock copolymer, a trace amount of lithium halide such as LiCl, LiBr or LiI will remain in the resulting block copolymer composition. It has already been known that such block copolymer compositions will cause subsequently formulated hot-melt-adhesive compositions to turn brown during the process performed at high temperature of 170-180.degree. C. It is also known that a block copolymer prepared by sequential polymerization or by coupling with epoxy resin will not cause discoloring in hot-melt processes. [WO 95/12644].
Another method of controlling the diblock content in block copolymer compositions was disclosed in such a manner that after preparing triblock and diblock copolymers in separate processes, the two copolymers are blended so as to obtain a desired diblock content [WO 89/08128].
Such method has an advantage to freely adjust the diblock content, but a large-scale blending facility is required.
More recently, a process of controlling the diblock content which does not use the methods of either coupling or blending was disclosed [WO 94/22931]. Said method modified the sequential polymerization process for the preparation of a linear triblock copolymer, where it comprises:
First step: a vinyl aromatic monomer and an organic lithium initiator are introduced into a reactor in the presence of a hydrocarbon solvent and reacted until the vinyl aromatic monomer is sufficiently polymerized;
Second step: A conjugated diene monomer is added to the above polymer and reacted until the mixture is sufficiently polymerized, thus synthesizing a living block copolymer in the type of vinyl aromatic block-conjugated diene block-Li;
Third step: Simultaneously both organolithium initiator and conjugated diene monomer are added to the above polymer solution and reacted until the mixture is sufficiently polymerized;
Fourth step: a vinyl aromatic monomer is added to the above polymer solution, thus obtaining copolymer product containing a triblock copolymer in the type of vinyl aromatic block-conjugated diene block-vinyl aromatic block-Li and a diblock polymer in the type of conjugated diene block-vinyl aromatic block-Li;
Fifth step: When the vinyl aromatic monomer is sufficiently polymerized, the reaction is completed with the addition of a polymerization terminator.
Some disadvantages of the above method have been recognized in that a) in the case of obtaining a composition with higher diblock content, the addition of relatively small amounts of initiator in the first ste,and difficult control of molecular may be responsible for prolonged reaction time, and b) since the rate of second initiation reaction which controls the uniformity of the diblock formation depends on the efficiency of organolithium initiator, a widely used initiator such as n-butyllithium, which is slower than sec-butyllithium in initiation reaction, may not be desirable in some conditions.